Bad Time Simulator

Bad Time Simulator is a fan-made recreation of the legendary Sans boss battle from Undertale, built to test reflexes and pattern recognition rather than button mashing. Players control a small heart trapped inside a black battle box while Sans unleashes an escalating barrage of bones, gravity-flip sections, and Gaster Blaster beams that must be dodged with pixel-precise movement. The game strips away dialogue and story to focus purely on the mechanical core of the fight, letting anyone jump straight into one of gaming's most notorious difficulty spikes and attempt a 1-HP run. Because every attack pattern is scripted but delivered at unforgiving speed, memorizing safe spots and staying calm under pressure matters more than fast reactions alone, and even a single mistimed step usually means starting the whole sequence over. The tight, arena-based bullet-hell structure has made it a favorite proving ground for speedrunners and Undertale fans alike. Players who enjoy this kind of relentless boss-rush pressure often also gravitate toward Cuphead, which channels the same trial-and-error dodging tension into a run-and-gun format.